Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of various advantages such as its non-impact, low noise characteristics and system simplicity. For these reasons, ink jet printers have achieved commercial success for home and office use and other areas.
Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the print head. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations.
In drop-on-demand ink jet printing, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink droplet that crosses the space between the print head and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets as the medium is moved relative to the print head.
In continuous stream or continuous ink jet printing, a pressurized ink source is used for producing a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a recording medium. Alternatively, deflected ink droplets may be allowed to strike the recording medium, while non-deflected ink droplets are collected in the ink capturing mechanism. While such continuous ink jet printing devices are faster than drop on demand devices and produce higher quality printed images and graphics, the electrostatic deflection mechanism they employ is expensive to manufacture and relatively fragile during operation.
Recently, a novel continuous ink jet printer system has been developed which renders the above-described electrostatic charging devices unnecessary and provides improved control of droplet formation. The system is disclosed in the commonly assigned U.S. Pat. No. 6,079,821 in which periodic application of weak heat pulses to the ink stream by a heater causes the ink stream to break up into a plurality of droplets synchronous with the applied heat pulses and at a position spaced from the nozzle. The droplets are deflected by heat pulses from a heater in a nozzle bore. This is referred to as asymmetrical application of heat pulses. The heat pulses deflect ink drops between a “print” direction (onto a recording medium), and a “non-print” direction (back into a “catcher”).
While such continuous ink jet printers utilizing asymmetrical application of heat have demonstrated many proven advantages over conventional ink jet printers utilizing electrostatic charging tunnels, a cost effective and reliable method and apparatus for controlling the heaters of the ink jet printer is required to ensure proper operation of the ink jet printer. Otherwise, misdirection of the ink droplets may occur which will detriment the print quality.